What a side sewer is, and who owns it
Quick answer: a side sewer is the private pipe from your building to the public sewer main, and in Seattle the property owner is responsible for maintaining and repairing it. City crews fix main-line blockages but do not clean up inside homes. This guide is for Seattle homeowners trying to understand a sewer backup.
Last updated: October 23, 2026
Seattle Public Utilities defines a side sewer as a privately owned pipe that carries wastewater from your building's sinks, drains and toilets to the public sewer mainline.
King County describes the same arrangement: side sewer pipes are found underneath your driveway or alleyway and carry wastewater to a sewer line under your local street, and property owners are responsible for installing, inspecting, maintaining and repairing them.
Sources: Seattle Public Utilities: Troubleshooting your sewer problem; Seattle Public Utilities: Taking care of your sewer pipe; King County: About wastewater backup protection for South Park.
Who handles what
| Part of the system | Who handles it |
|---|---|
| Side sewer (building to public main) | The property owner |
| Public sewer main in the street | Seattle Public Utilities (report at 206-386-1800) |
| Cleanup inside your home | The homeowner; city crews do not enter homes |
| Backwater valve maintenance | The property owner |
Sources: Seattle Public Utilities: Clean up the backup; Seattle Public Utilities: Sewer-backup prevention, backwater valves (policy DWW-351).
Shared side sewers
Some properties share a side sewer with a neighbor, while others have separate pipes. Seattle Public Utilities explains both arrangements in its guide, and recommends finding out which applies to your property. SPU also recommends having your side sewer inspected, for example by a contractor, to find problems early.
Source: Seattle Public Utilities: Taking care of your sewer pipe.
After a backup: what to do
- Stop using water and keep people and pets out of the area.
- Call Seattle Public Utilities if you suspect the main line is blocked.
- Contact a side sewer contractor to inspect and repair breaks or blockages in your pipe.
- Document the damage and call your insurer.
- Ask about a backwater valve to prevent a repeat. See our backwater valve guide.
For the cleanup itself, see sewage backup cleanup in Seattle.
Sources: Seattle Public Utilities: Troubleshooting your sewer problem; Seattle Public Utilities: Clean up the backup.
Side sewer questions
What is a side sewer?
A side sewer is the privately owned pipe that carries wastewater from your building's sinks, drains and toilets to the public sewer main, according to Seattle Public Utilities.
Who is responsible for the side sewer in Seattle?
The property owner. Seattle Public Utilities says property owners are responsible for maintaining and repairing their side sewers, and King County says the same for owners in the Seattle area.
Can I share a side sewer with a neighbor?
Yes. Seattle Public Utilities notes that some properties share a side sewer, so you may share one with your neighbor. Find out which applies to your property before a problem happens.
Who do I call for a blockage in the street?
For problems in the main sewer line you can call Seattle Public Utilities at 206-386-1800. Its crews respond to main-line blockages but do not enter homes to clean up a backup.
Does home insurance cover a side sewer backup?
Not under a standard policy. The Washington Insurance Commissioner says sewer backup coverage is an optional add-on, so ask your agent about a sewer backup endorsement.